Speedo Re-calibration.

[5isalive]

Hello MX5cartalk community,

I have a 2003 NB Series 3 that has 17" wheels fitted and I've also recently had the diff changed out from the 3:6 ratio to a 4:1.

The problem now is, I have a speedo that reads incorrectly throughout the whole range.

I only noticed this when I was driving home the other night. I was on the highway sitting on 110kph, which is what my speedo was indicating, but I was then being passed by semis and old people towing caravans. Either I was doing the correct speed or everyone else was doing at least 10kph over.

Thinking it was more than likely me at fault, I tagged onto a few different vehicles and most of them had me traveling at around 121kph.

Figuring the diff ratio was the culprit, I thought if I just drove at 10kph faster this would make up the shortfall, and probably solve the issue, at least in the short term.

A friend suggested I download a "heads up display" app to my phone, which I did, and this verified the differences in speed I'd been getting when following other vehicles, but most of all, it showed that I was getting really oddball figures.

For example, if the limit was 110kph, I need to do be doing 121kph, at 100kph it's around 108kph, 80kph is about 93kph and so on and so forth. As you can imagine, this is a real pain in the dot when trying to drive safely and attempting to calculate 9 different speed limits at the same time, it's really doing my head in.

So, I'm in a bit of a dilema. I'm aware that there's a coloured "nylon gear" in the diff that can be changed to adjust the difference of ratios, but due to the larger wheels, I don't know if this would actually fix the problem because of the varied readings I'm getting, so I was wondering, can I get the speedo re-calibrated to show the actual speed? And if so, how?

My wife suggested using the HUD on the phone, and I will in the meanwhile, but I'd much rather find a solution so I can use my speedo the way it's meant to be.

Please, if you could keep any replies suited to someone with an equivilent mechanical knowledge to that of a 5th grader, I would very much appreciate it. As usual, thanks for any assistance sent my way.

Cheers, Wayne.

[project.r.racing]

Because of the diff.

I think now you need a speedo sensor from the car the new diff came from.

[ianoz]

as above - either the gear to suit the 4.1 (though you'll still be out due to the larger wheels), or do it electronically.

NB's have electric speedos, so you can put a module in line to adjust the 'revs' the speedo 'sees'.
example - https://www.jaycar.com.au/corrector-speedo-module/p/AA0376

bit of stuffing around to get it calibrated, but when I priced a gear for mine it was about $150ish...

don't understand why the variation jumps around though. It should be the same % difference across the range and on your figures it jumps around between 8% (100-108) and 16% (80-93)

[5isalive]

Thanks for the replies so far.

1. I don't think I'll be able to source the sensor from the donor car as it's in Victoria, and even if I did, I wouldn't know how to fix it.

2. I can't understand why the speedo fluctuates so much either. I guess the wheels make a few kph difference and the diff on top of that puts it out of whack a bit more, combining to make an odd set up.

I could understand if it were a steady 10kph throughout the range, that would make it so much easier to adjust to mentally, but when it's 7 or 8 kph difference at every 10km marker on the speedo it's ridiculous, especially with the unforgiving Police nowadays.

I have about 2 points left, so the slightest incursion will have me sitting at least the next year out.

Would any auto electrician be able to fix this or is it a specialist thing?

Cheers.

[greenMachine]

Yes, changing the diff ratio throws out the speedo, by the % difference in the diff ratios, the bigger numerical ratio being the slowest (but giving you the punchiest acceleration). So you are going slower than your speedo says (because it doesn't know you have changed ratios). Going from a 3.6 to a 4.1 is therefore making your speedo about 11% faster (ie you are travelling 11% slower than the speedo says) at ALL speeds.

Bigger wheels (going from 16s to 17s) may affect the speedo, but if the tyres are a lower profile, such that the circumference is the same as the 16s that came with the car, there will be no change. If the circumference is greater, the car will be faster than the speedo says (ignoring the built-in error margin that is in all speedos); smaller, and you will be slower.

Just taking your 121/110kmh example that suggests that your tyres are the same circumference as the 16s, near enough, as it seems you are travelling 10% slower than your speedo says.

My advice is to live with it. I did. For many years I had a 6-speed in my NB8A (4.1 diff), giving me gearing that put my speedo out 11% just like you, and I just adjusted to it, and took the necessary precautions where speed limits were concerned.

You could get the gearbox sender/gear (the sender is on the gearbox, not the diff) from a car running a 4.1 - mainly NB8As, which ran 5-speeds and I don't know if the senders from the 6-speeds and the 5-speeds are interchangeable. If not, a 6-speed 4.1 sender may be available off a JDM car perhaps, or even a US or European one. Someone may know, but I haven't found a listing of what diff ratios were available where (for 6-speed NBs). I don't know if you can get gears separate to the sender unit. The electronic device suggested by Ian may suit you better.

The other way may be to move the needle in the speedo on its shaft, you would have to experiment, and I have never tried this so I am not sure it is even possible on an NB, probably would on an NA but that's no help to you.

When you ask about recalibration, these are the three ways - 4.1 sender/gear; move the speedo needle; or use an electronic device to 'tune' the signal from the sender. If there are others, I don't know of them (well you could adjust tyre sizes I suppose).

I hope that helps.

[hks_kansei]

As far as the Speedo sensor goes, you can buy one from any 5speed Aussie NB8A (1998-2000)

Fitting the sensor is as easy as getting under the car, removing a single 10mm bolt, unplug the old sensor and slide it out of the gearbox, slide he new one in, put the bolt back and plug the wires in.

The sensor just sticks out the side of the box, and is designed for simple replacement.

[rascal]

just dont take the wrong bolt out........

[bruce]

I think the Jaycar device is best for you. Mighty Car Mods have a video doing an install (?).

[project.r.racing]

1. I don't think I'll be able to source the sensor from the donor car as it's in Victoria, and even if I did, I wouldn't know how to fix it.

Yet the diff managed to get itself onto the car all by itself?

If you dont know how to do it. Then get the person that attached the diff to your car to do it.

[5isalive]

Sorry for not responding to the replies I've received so far. As most of you would know, the site has been down for a few days.

Anyhow, thank you all for your suggestions. I may have to try the sensor on the side of the gearbox first up. That sounds like the easiest fix... I hope.

If that doesn't improve things, the device that recalibrates the speedo may be the way to go. I'm really not confident enough to try moving the needle on the guage though, so I may have to give that one a miss for now.

What was said about the 17" wheels makes perfect sence to me now. I never took into account that they were fitted with low profile tyres. This would explain why my speed didn't appear to be effected with the 17's, which were on the car when I bought it.

As for the witty comment about "the diff getting onto the car all by itself". The fact is, if I knew how to fix the problem, I wouldn't be here. I'm a musician, not a mechanic, however, if you want to know how to play a D#7sus4 chord, I'm your man.

Once again, thanks for the helpful tips. They certainly give me a good place to start. Please keep them coming. If anyone has any other ideas or suggestions, I would really appreciate hearing them.

Cheers, Wayne.

[project.r.racing]

Maybe you missed my point from my "witty" comment.

You aren't mechanical minded but yet you used an avenue to get the new diff attached to the car.

I would use that same avenue to source and attach the new diff sensor.

[5isalive]

Sorry project.r.racing if I misread your comment. It very much sounded like sarcasm to me. If it wasn't, please accept my apology.
Cheers, Wayne.

[project.r.racing]

No it was sarcasm, but the point made in the comment was valid.